1. Field of the Invention
The invention relates to the production of aluminium by fused bath electrolysis using the Hall-Héroult process. It is used particularly for controlling additions of powder materials into an electrolyte bath of electrolytic cells.
2. Description of Related Art
The operation of a cell for the production of aluminium by fused bath electrolysis of alumina dissolved in a cryolite based bath causes a permanent change in the composition of the bath. Firstly, alumina is consumed by the electrolysis reactions, and secondly the quantity and composition of the bath are gradually modified by secondary mechanisms such as absorption of cryolite constituents by the walls of the cell or decomposition of fluorinated constituents by anode effects. Consequently, alumina and bath compounds such as cryolite (Na3AlF6) or aluminium fluoride (AlF3) have to be added regularly in order to stabilise operation parameters of the cell. The purpose of this stabilisation is in particular to achieve the highest possible Faraday efficiency and to prevent anode effects caused by a shortage of alumina in the bath and the accumulation of alumina “sludge” at the bottom of the pots caused by excess alumina.
The alumina and bath compounds are usually added into the bath in the form of a powder. Several methods and devices are known for automatically “feeding” electrolytic cells with powder materials in a regulated manner. For example, the following patent applications in the name of Aluminium Pechiney describe methods for regulating additions of alumina, aluminium fluoride or other: FR 2 749 858 (corresponding to U.S. Pat. No. 6,033,550), FR 2 581 660 (corresponding to U.S. Pat. No. 4,654,129), FR 2 487 386 (corresponding to U.S. Pat. No. 4,431,491), FR 2 620 738 (corresponding to U.S. Pat. No. 4,867,851) and FR 2 821 363.
In order to be able to add powder material into the electrolyte bath, electrolytic cells are equipped with one or several powder material distributors associated with a device for boring the alumina and solidified electrolyte crust that covers the bath surface during normal operation. The boring device usually comprises a jack and a crustbreaker (or “plunger”) fixed to the rod of the jack. The plunger is lowered when the jack is actuated and breaks the alumina and solidified bath crust. This operation may be repeated several times and regularly so as to keep the hole through which the powder material is added open. Patent applications FR 1 457 746 (corresponding to GB patent 1 091 373) and FR 2 504 158 (corresponding to U.S. Pat. No. 4,435,255) and U.S. Pat. No. 3,400,062 describe such devices.
However in some conditions, the boring device cannot guarantee that powder material can be added into the bath. In particular, the hole can sometimes get plugged by an alumina block that becomes agglomerated with the solid bath, which hinders the “feed” of powder materials into the bath. The boring device may also be defective. It has been proposed to deal with this type of operation anomaly by making electrical measurements to detect whether or not the plunger is actually in contact with the electrolyte. For example, in patent application FR 2 483 965 (corresponding to U.S. Pat. No. 4,377,452) in the name of Aluminium Pechiney, the contact between the electrolyte and the plunger is detected by an electrical measurement between the crustbreaker and the cathode. If no contact with the electrolyte is detected after a predetermined lapse of time, the system for example gives an order to lift the plunger or stop the feed. This method has the disadvantage that it is sensitive to voltage fluctuations in the cell, particularly during anode effects. American patent U.S. Pat. No. 4,563,255 in the name of Swiss Aluminium describes a similar but more complex solution that uses impedance measurements.
The applicant searched for means of detecting and taking account of operation anomalies in the feed of powder materials to an electrolytic cell that do not depend on electrical measurements made directly on the cell.